1. Field of the Invention
The present invention relates to techniques for refreshing the inputs of a receiver. More specifically, the present invention relates to a method and apparatus for refreshing the DC voltage-level of the inputs of a differential receiver.
2. Related Art
Advances in semiconductor technology presently make it possible to integrate large-scale systems, including tens of millions of transistors, onto a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs, because fewer semiconductor chips are required to perform a given computational task.
However, these semiconductor chips still need to communicate with other chips, and unfortunately, these advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated into a printed circuit board which contains multiple layers of signal lines for inter-chip communication. However, signal lines are typically 100 to 1000 times denser in a semiconductor chip than a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem is creating a bottleneck that is expected to worsen as semiconductor integration densities continue to increase.
Furthermore, traditional conductive connections have limitations. For example, wire-bonds or solder-balls can stop conducting due to environmental effects such as oxidation and mechanical stress. They also have undesirable properties, such as low density, low yield, and permanent attachment.
One solution to the limitations of conductive connections is to replace the direct conductive coupling with direct chip-to-chip capacitive coupling, referred to as “Proximity Communication,” which significantly increases bandwidth and reduces power consumption when communicating between chips, while also avoiding the need for permanent chip attachments. The capacitive interface used to couple integrated circuit chips together blocks the DC component of the signal.
Blocking the DC component of the signal presents challenges as well as benefits. One challenge is that in order to extract the DC level and properly recover the transmitted information, either the receiver or the spectral content of the signal must be modified. One advantage is that by coupling the signal through a capacitor the DC voltage between the transmitter and the receiver is isolated, thereby allowing the receiver to be set to its optimal gain point regardless of DC voltage at the output of the transmitter.
FIG. 1A illustrates a differential communication channel. It contains transmitter 102, communication channels 104 and 106, and receiver 108. txdata and txdatab are transmitted from transmitter 102 through channels 104 and 106, respectively. Input signals in and inb (corresponding to txdata and txdatab) are received at receiver 108, which then amplifies these signals to produce rxdata and rxdatab, respectively.
FIG. 1B illustrates a differential communication channel using series capacitors 110 and 112 as communication channels 104 and 106, respectively.
More general channel models can include delay or loss elements. For example, a channel model can include a transmission line and series capacitors at the input, output, or both the input and the output of the transmission line. Additionally, the channel model can also include parasitic loss elements such as resistors in series with the transmission line, capacitors shunting the input and output nodes to an AC ground, or inductors in series with the transmission line.
A series capacitor in the conmmunication channel blocks the DC component of a signal. Therefore, a receiver which draws no DC current at the inputs must set its own DC operating point. FIG. 1C illustrates a circuit used to set the DC operating point of a receiver. It contains capacitor 118, which blocks the DC voltage-level generated by the transmitter. PMOS transistor 114 and NMOS transistor 116 form an inverter powered by voltages VLO and VHI which is used in a feedback-loop such that the circuit remembers the previous value transmitted. For example, if in is a 1, then inb is a 0. Note that PMOS transistor 114 and NMOS transistor 116 are sized such that they are weak compared to capacitor 118. In other words, the output of the weak inverter formed by PMOS transistor 114 and NMOS transistor 116 weakly holds in to a value of VHI or VLO. Since in is held weakly, a fast switching of txdata transmitted through capacitor 118 can switch the state of the inverter, if appropriate. Note that a slow switching of txdata cannot switch the state of the inverter. On the other hand, noise that may be coupled onto in from power supply bounces, other signals, thermal noise, 1/f noise, or other noise sources, is rejected by holding in strongly enough to overpower the noise.
Note that VHI is set such that it equals VTHRESHOLD+ΔV and VLO is set such that it equals VTHRESHOLD−ΔV. VTHRESHOLD is the switching threshold of inverter 120. ΔV is a small voltage such that VTHRESHOLD+ΔV or VTHRESHOLD−ΔV will cause inverter 120 to switch to the low voltage and high voltage of inverter 120, respectively.
Unfortunately, the choice of ΔV must carefully balance the requirements to reject small perturbations on IN as noise, while not rejecting transitions caused by txdata switching and coupling through capacitor 118.
FIG. 1D illustrates a circuit that overcomes this balancing limitation by simply biasing the in node to VTHRESHOLD through large-valued resistor 122. Resistor 122 is chosen to be large so that it averages the effect of many bits. Unfortunately, even though the DC voltage-level of the input of the receiver can be set, an unbalanced string of data (mostly “1”s or mostly “0”s) causes an imbalance in the DC level of the receiver, eventually making data recovery impossible.
One solution to this problem is to encode the data to balance the DC signal content. Encoding techniques such as balanced codes and scramblers can be used. In balanced codes, a set of N data bits are mapped onto M data bits, where M>N so that M/2 bits are “1.” Unfortunately, the code reduces the useable bandwidth by (M−N)/M. In scramblers, the data is XOR'ed with a pseudorandom bit-sequence. Although scramblers do not reduce the useable bandwidth, if the data signal correlates with the scrambler sequence, scramblers cannot guarantee that all of the DC signal content is removed.
Another solution is to create electrically floating nodes at the input of the receiver. These nodes have no conductive discharge path. Hence, they can maintain their programmed voltage for years. Unfortunately, the mechanism which programs these nodes often requires special fabrication techniques similar to those used to create EEPROM devices. These special fabrication techniques may not be available in a CMOS fabrication technology.
Hence, what is needed is a receiver that can receive capacitively-coupled signals without the problems described above.